1. Field of the Invention
This invention relates to a method of suppressing occurrence of a natural convection in a fluid in a cylindrical vessel and a method of growing a compound semiconductor crystal of Group II-VI elements of Periodic Table such as ZnSe utilizing the method of suppressing occurrence of a natural convection.
2. Description of the Prior Art
When there is a density gradient in a fluid in a cylindrical vessel, it is desirable to arrange the fluid in micro gravity so as to suppress natural convection occurring in the fluid. The environment under micro gravity can be realized by subjecting the fluid itself to free fall or arranging it outside the gravity zone utilizing satellite orbit.
A method comprising growing a compound semiconductor crystal of Group II-VI elements of Periodic Table such as ZnSe, ZnS, CdS, etc. by a chemical vapor transport method using iodine as a transport agent has widely been used, but in general, it is said to be difficult to obtain a single crystal under such a condition that natural convection in an ampoule is strengthened. For example, it has been reported in xe2x80x9cJ. Crystal Growth 146 (1995) 53xe2x80x9d that in the case of growing ZnSe crystal, its single crystal can be obtained only under such a condition that a gas pressure in an ampoule is small and natural convection is thus hard to occur. Furthermore, it has been proposed in Japanese Patent Laid-Open Publication No. 4894/1993 to provide an internal structure for preventing occurrence of natural convection and obtaining a single crystal between a raw material charged zone and crystal growth zone in an ampoule.
The above described method of suppressing occurrence of natural convection in micro gravity, however, needs an expensive installation whose application is largely limited. For example, the micro gravity by a falling tower is lasting only in a very short time, e.g. about 10 seconds. Consequently, this installation is limited in application thereof and cannot be applied to a method needing several hours to several ten days, such as a method for the growth of a crystal.
In a method for the growth of a compound semiconductor crystal of Group II-VI elements of Periodic Table by a chemical vapor transport method using iodine as a transport agent, for example, an internal pressure in an ampoule is generally high, for example, several atm., and natural convection tends to occur. Further, it is noted that when the natural convection is strong, a grown crystal tends to be polycrystalline and growth of a single crystal is difficult. When an inner diameter of a growth ampoule is increased, in addition, the convection is strong and accordingly, even if the crystal growth of a compound semiconductor crystal of Group II-VI elements of Periodic Table is carried out, it is difficult to obtain a large-sized single crystal.
In a method comprising providing an internal structure for preventing occurrence of gaseous convection between a raw material charged zone and crystal growth zone in an ampoule, as disclosed in Japanese Patent Laid-Open Publication No. 4894/1993, it is required to enlarge the crystal growth zone to grow a large-sized crystal and consequently, a new gaseous convection occurs in the crystal growth zone so that sufficient convection suppressing effect cannot be maintained.
It is an object of the present invention to provide a method of suppressing convection of a fluid in a cylindrical vessel by means of realizing an environment under micro gravity which can be maintained for such a long time that growth of a large-sized crystal be economically effected, whereby the above described problems can be solved.
It is another object of the present invention to provide a method of growing a compound semiconductor crystal of Group II-VI elements of Periodic Table by utilizing the method of suppressing convection of a fluid in a vessel.
These objects can be attained by a method of suppressing convection of a fluid in a cylindrical vessel, when a density gradient due to difference in temperature, concentration or partial pressure is added to the gas or liquid filled in the cylindrical vessel along the central axis of the vessel, characterized by maintaining horizontal the vessel and rotating it around the central axis.